The present invention relates to a Cr-containing titanium nitride film having an excellent wear resistance and a low frictional coefficient. More specifically, it relates to a Cr-containing titanium nitride film suitable for a slide member such as a bearing or a seal used in rotary machines operated at a high temperature, such as a steam turbine and a gas turbine.
In order to increase a wear resistance or a corrosion resistance of a bearing or seal member formed of a metallic material, application of a ceramic coating to its surface has been widely conducted. Examples of a material used in the ceramic coating include titanium nitride (TiN), titanium carbide (TiC), chromium nitride (CrN), boron nitride (BN) and diamond-like carbon (DLC). Of these, TiN and CrN have been already widely industrialized, and have been used in a mold and a cutting tool as a hard film.
As a method for forming such a hard film, a surface modification technique such as an ion plating method typified by a PVD method or a CVD method, a sputter deposition method, a plasma CVD method or an ion implantation method has been studied. Especially, a dynamic mixing (DM) method which is a combination of a vacuum deposition method and an ion implantation technique has attracted much interest as a film-forming technique which provides an excellent adhesion with a substrate and enables formation of a product at a low temperature.
Titanium-nitride (TiN), one of ceramic coating materials which have been widely used is known to be a typical substance that forms an interstitial compound having a face-centered cubic crystal structure. In TiN, nitrogen enters a Ti lattice as an interstitial solid solution to give an NaCl-type crystal structure. TiNx can take a wide composition zone of 0.8 less than xc3x97 less than 1.16. It is known that when x is varied within this composition zone, a lattice constant of TiN is changed. Since a TiN film is excellent in wear resistance and corrosion resistance, it has been used in some bearing or seal members.
By the way, in rotary machines operated at a high temperature, such as a steam turbine and a gas turbine, the development of a hard film having an excellent wear resistance, an excellent high-temperature corrosion resistance and a high slidability has been in demand with the increase in operation temperature of rotary machines. A Ti film is considered to be employed in this use. However, it has been found from the past experiments that when a TiN film is exposed to a high-temperature atmosphere or a high-temperature water vapor, a high-temperature corrosion resistance of TiN itself is not satisfactory and is problematic in durability. Accordingly, the existing TiN film could not have exhibited satisfactory sliding characteristics in this use.
The invention has been made to solve these problems, and it aims to provide a Cr-containing titanium nitride film which has an excellent wear resistance and an excellent slidability with a low frictional coefficient and which can be improved in high-temperature corrosion resistance.
In order to attain this aim, the first embodiment of the invention is a Cr-containing titanium nitride film which is composed of a nitride containing Ti and Cr as main components, the crystal particles thereof having a face-centered cubic structure, and the crystal thereof being highly oriented toward (200) face.
For improving the high-temperature corrosion resistance and the oxidation resistance of the titanium nitride film, the present inventors have tried to obtain a nitride thin film containing elements other than Ti and N, and have developed a technology of forming such a nitride thin film. That is, they have conducted investigations on a technology of forming a nitride thin film containing elements other than Ti and N in consideration of improving the high-temperature corrosion resistance of the titanium nitride thin film without impairing the high sliding characteristics (wear resistance, low frictional coefficient) inherent in the titanium nitride thin film. Consequently, it has been found that in a nitride film containing titanium nitride as a main component and further containing Cr, good frictional wear characteristics are exhibited when the crystal structure is a face-centered cubic (fcc) structure and the crystal particles are highly oriented toward (200) face. In this case, the resulting thin film has a Vickers hardness of 2,500 or more.
The second embodiment of the invention is a Cr-containing titanium nitride film, wherein Cr-containing titanium nitride has the following chemical composition,
Ti(100-x1)Crx nitride
wherein 30 at.% less than xc3x97xe2x89xa660 at.%, and impurities are oxygen and carbon, the content of oxygen or carbon being several at.%.
It is advisable that the Cr-containing titanium nitride film in the first and second embodiments of the invention is formed by a dynamic mixing (DM) method such that nitrogen is ion-implanted while vacuum-depositing Ti as a metallic element and an additional element onto a metallic substrate. This method can form a film having a high adhesion with the substrate and produce a product at a low temperature. It is advisable for maintaining an adhesion to use stainless steel such as an SUS 420J2 steel or an SUS 630 steel, an Ni alloy such as an INCOLOY 909 alloy or a Hastelloy alloy, which has a coefficient of thermal expansion of 11xc3x9710xe2x88x926 or less, as a substrate.
It is preferable that an accelerating voltage of ion beams is 40 kV or less. When it is more than 40 kV, an ion beam acceleration device becomes large-scaled, inviting high processing costs or requiring a measure against radiation. Further, when an energy of applying ion beams is less than 1 kV, an adhesion with a substrate is insufficient, and a hard film suitable for a high-temperature slide member is not obtained.
From the results of measurement by an X-ray diffraction (XRD) method, it is presumed that the size of crystal particles of the nitride thin film is preferably several nanometers to 100 nanometers. It is advisable that the thickness of the hard film formed is less than several tens of micrometers in consideration of various factors such as processing costs and a residual stress of the film. However, various thicknesses are available according to the usage.
The ratio of the additional element (Cr) can be determined by controlling the evaporation rates of Ti and the additional element (Cr) in the DM method. In TiN, nitrogen enters a Ti lattic as an interstitial solid solution, and a face-centered cubic crystal structure is provided. The crystal particles can be oriented toward (200) face by controlling the irradiation conditions of nitrogen ion beams, for example, an accelerating voltage of ion, a current density, an application energy (W/cm2) and an irradiation angle in the DM method. As the Cr content is increased, the crystal is highly oriented toward TiN (200) face.
The third embodiment of the invention is a process for producing the Cr-containing titanium nitride film in the first or second embodiment of the invention, which comprises vacuum-depositing Cr and Ti on a substrate simultaneously, and applying ion beams made mainly of nitrogen to form Cr-containing titanium nitride.
The fourth embodiment of the invention is a slide member comprising a combination of a movable member and a stationary member, one of the movable member and the stationary member being made of a metal, and the other being made of a carbon-containing material, wherein the Cr-containing titanium nitride film in the first or second embodiment of the invention is formed on the slide surface of the movable member or the stationary member made of the metal.
The invention is specifically described below by referring to the drawings. The following description is directed to one specific mode of the invention, and the invention is not limited thereto.